ATTR

NAME

attr - Extended attributes

DESCRIPTION

Extended attributes are name:value pairs associated permanently with
files and directories, similar to the environment strings associated
with a process.
An attribute may be defined or undefined.
If it is defined, its value may be empty or non-empty.

Extended attributes are extensions to the normal attributes which are
associated with all inodes in the system (i.e. the
stat(2)
data).
They are often used to provide additional functionality
to a filesystem - for example, additional security features such as
Access Control Lists (ACLs) may be implemented using extended attributes.

Users with search access to a file or directory may retrieve a list of
attribute names defined for that file or directory.

Extended attributes are accessed as atomic objects.
Reading retrieves the whole value of an attribute and stores it in a buffer.
Writing replaces any previous value with the new value.

Space consumed for extended attributes is counted towards the disk quotas
of the file owner and file group.

Currently, support for extended attributes is implemented on Linux by the
ext2, ext3, ext4, XFS, JFS and reiserfs filesystems.

EXTENDED ATTRIBUTE NAMESPACES

Attribute names are zero-terminated strings.
The attribute name is always specified in the fully qualified
namespace.attribute
form, eg.
user.mime_type,
trusted.md5sum,
system.posix_acl_access,
or
security.selinux.

The namespace mechanism is used to define different classes of extended
attributes.
These different classes exist for several reasons, e.g. the permissions
and capabilities required for manipulating extended attributes of one
namespace may differ to another.

Currently the
security,
system,
trusted,
and
user
extended attribute classes are defined as described below. Additional
classes may be added in the future.

Extended security attributes

The security attribute namespace is used by kernel security modules,
such as Security Enhanced Linux.
Read and write access permissions to security attributes depend on the
policy implemented for each security attribute by the security module.
When no security module is loaded, all processes have read access to
extended security attributes, and write access is limited to processes
that have the CAP_SYS_ADMIN capability.

Extended system attributes

Extended system attributes are used by the kernel to store system
objects such as Access Control Lists and Capabilities. Read and write
access permissions to system attributes depend on the policy implemented
for each system attribute implemented by filesystems in the kernel.

Trusted extended attributes

Trusted extended attributes are visible and accessible only to processes that
have the CAP_SYS_ADMIN capability (the super user usually has this
capability).
Attributes in this class are used to implement mechanisms in user
space (i.e., outside the kernel) which keep information in extended attributes
to which ordinary processes should not have access.

Extended user attributes

Extended user attributes may be assigned to files and directories for
storing arbitrary additional information such as the mime type,
character set or encoding of a file. The access permissions for user
attributes are defined by the file permission bits.

The file permission bits of regular files and directories are
interpreted differently from the file permission bits of special files
and symbolic links. For regular files and directories the file
permission bits define access to the file's contents, while for device special
files they define access to the device described by the special file.
The file permissions of symbolic links are not used in access
checks. These differences would allow users to consume filesystem resources in
a way not controllable by disk quotas for group or world writable special files and directories.

For this reason, extended user attributes are only allowed for regular files and directories, and access to extended user attributes is restricted to the
owner and to users with appropriate capabilities for directories with the
sticky bit set (see the
chmod(1)
manual page for an explanation of Sticky Directories).

FILESYSTEM DIFFERENCES

The kernel and the filesystem may place limits on the maximum number
and size of extended attributes that can be associated with a file.
Some file systems, such as ext2/3 and reiserfs, require the filesystem
to be mounted with the
user_xattr
mount option in order for extended user attributes to be used.

In the current ext2, ext3 and ext4 filesystem implementations, each
extended attribute must fit on a single filesystem block (1024, 2048
or 4096 bytes, depending on the block size specified when the
filesystem was created).

In the XFS and reiserfs filesystem implementations, there is no
practical limit on the number or size of extended attributes
associated with a file, and the algorithms used to store extended
attribute information on disk are scalable.

In the JFS filesystem implementation, names can be up to 255 bytes and
values up to 65,535 bytes.

ADDITIONAL NOTES

Since the filesystems on which extended attributes are stored might also
be used on architectures with a different byte order and machine word
size, care should be taken to store attribute values in an architecture
independent format.